irq_remapping: Introduce new interfaces to support hierarchical irqdomains

Introduce new interfaces for interrupt remapping drivers to support
hierarchical irqdomains:

1) irq_remapping_get_ir_irq_domain(): get irqdomain associated with an
interrupt remapping unit. IOAPIC/HPET drivers use this interface to
get parent interrupt remapping irqdomain.

2) irq_remapping_get_irq_domain(): get irqdomain for an IRQ allocation.
This is mainly used to support MSI irqdomain. We must build one MSI
irqdomain for each interrupt remapping unit. MSI driver calls this
interface to get MSI irqdomain associated with an IR irqdomain which
manages the PCI devices. In a further step we will store the irqdomain
pointer in the device struct to avoid this call in the irq allocation
path.